The treatment of sewage by aeration may be carried out with a series of discs or paddles mounted on a horizontal shaft rotating just above the surface of the sewage. A particular type of sewage treatment plant for the extended aeration of the sewage or which may carry out the activated sludge process with such discs comprises a series of channels which are usually concentric for construction economy. The discs have two main functions, namely to inject diffused air into the mixed liquor in the channels and to circulate the mixed liquor in the channels to prevent settling and provide mixing. As each disc rotates, the mixed liquor which wets the disc is also aerated by the contact aeration process. This is of a relatively lesser effect but is significant to the present invention.
The present invention is an improvement over a preferred type of aeration disc which has a multiplicity of perforations of a certain size. Such discs may be designed to operate with great efficiency.
As each disc rotates, each portion having a given perforation or hole is repeatedly exposed to the air and then submerged in the mixed liquor. The rotational speed of the disc is such that the air in the hole is carried to below the surface of the mixed liquor where it is displaced by the liquor and optimally leaves the hole as a series of very small bubbles. Some of the oxygen of the air at the interface of each air bubble and the liquor is dissolved in the water before the bubble breaks at the liquid surface. The mentioned efficiency is the rate of such oxygen transfer relative to the horsepower required to rotate the shaft and may be precisely measured.
It is well known that the particularly small size of the bubbles is an important factor because of their greater surface area for a given volume of air and that if the air in leaving a given hole is sheared into small bubbles by the liquor passing over the disc, its efficiency is greatly improved.
The exact flow pattern of the liquor which displaces the air from a given hole has not been determined, but can be conjectured. Assuming absolute symmetry, liquor enters the leading portion of the hole in opposed directions from both sides of the disc at a given rate and the air similarly leaves the trailing portion of the hole in opposite directions from both sides of the disc at the same rate. Assuming no symmetry, the liquor enters the hole from one or the other side of the disc and pushes the air out of the hole on the opposite side of the disc. Whatever occurs, of course, is generally of some intermediate nature. In any case, the air should be displaced well before the hole again emerges from the liquor and it may be that some movement of the liquor through the hole occurs during each submersion. While such flow is conjectured, it seems to be indicated by the problem which has developed in the treatment of sewage having an appreciable amount of suspended material such as hair and stringy debris. Apparently with each submersion, there is a significant flow of liquor through the hole probably after the air has been displaced and no doubt most of the material carried by the flow is carried through the hole. Nonetheless, some amount, probably first the longer hairs and the like, initially become draped over both sides or faces of the disc. This is indicated by the fact that after several weeks of operation many holes are plugged by hairs, strings and the like, although significantly fewer holes appear to be partially plugged at any given time. The plugged and partially plugged holes may be readily observed when the discs are not rotating and of course the effect is to drastically reduce the efficiency of the disc. Accordingly, the object of the present invention is to prevent such plugging of the holes without in any way decreasing the oxygen transfer efficiency of the disc, and possibly with some increase in such efficiency. Another object of the invention is to provide an aeration disc with recesses in each face which provide high efficiency but which are resistant to plugging by suspended material of any sort.
Another object of the invention is to provide recesses in an aeration disc which may be further provided also with projecting nodules which also entrain air and in particular increase the impelling action of the discs. Another object of the invention is to increase the surface area of the disc and the efficiency of the disc by contact aeration, at least to the extent that the recesses of a given disc may have somewhat less total volume than similar holes would have.